Commercial production of polyesters such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and other polyesters conventionally synthesized from diols and dicarboxylic acids reaches billions of pounds every year. Polyesters are used for manufacturing of a wide variety of products ranging from fabrics to bottles and surgical materials.
Traditional methods for production of polyester require polycondensation of diols with dicarboxylic acids or their lower alkyl esters. These reactions proceed with elimination of low molecular weight by-products such as water or alcohols. Large amounts of low molecular weight compounds produced during polycondensation of polyesters require one or more steps of removal from the polymers. Thus reduction of the amount of water or alcohols during polycondensation of polyesters would significantly simplify the process and reduce the processing cost. Also, reactions involving diacids and diols require an excess of diol to drive the reaction equilibrium forward, thus increasing the reaction cost.
Synthesis of polyester from compounds containing one hydroxy group and one carboxy group such as 4-hydroxybenzoic acid, or 6-hydroxy-2-naphthoic acid disclosed in U.S. Pat. Nos. 4,431,770 and 4,161,470 both issued to Calundann instead of diacids and diols also has an advantage over the diol-diacid process since it does not require excess amounts of diols. However, condensation of these reactants to produce polyesters still produces one molecule of water for each ester link in the polyester.
Synthesis of polyesters from carboxy mono-hydroxyesters, i.e. monoesters containing both a hydroxy group and a carboxylic acid group, simplifies the synthesis of polyesters. For example, one of the most widely produced polyesters, polyethylene terephthalate (PET) can be produced from the monomer, mono-2-hydroxyethyl terephthalate (MHET), which is typically produced from starting reactant materials of ethylene glycol and terephthalic acid. Recently, the industry has taken a closer look at the reaction mechanisms of producing MHET to determine how to maximize the production of this monomer over other hydroxyethyl terephthalate monomers.
Other starting materials for producing mono-hydroxyesters, are known in the art. U.S. Pat. No. 2,937,297 to Katzschmann et al. discloses production of MHET by re-esterifying an alkali metal salt of the monoesters of terephthalic acid with mono-valent alcohols. The process is complicated and quite costly. Various processes for the esterification of carboxylic acids have been studied in great detail in liquid systems (Haslam, Tetrahedron, 1980, 36, 2409-2433). However, esterification of acids in liquid systems is usually expensive due to the high cost of solvents. Also, the reaction usually requires further purification of the monoesters prior to their use as monomers for the synthesis of polyesters.
Recently, it has been reported that supercritical fluids can be used as effective solvents for use in heterogeneous reactions with the reaction product being present in the fluid phase (Dillow, A. et al, Ind. Eng. Chem. Res. 1996, 35, 1801-86, Chandler, K., et al, Ind. Eng. Chem. Res. 1998, 37, 3252-59). Supercritical fluids (SCF) are very compressible, so they undergo large changes in density, and thus solvation strength, with small changes in temperature or pressure (Gurdial, G., and Foster, N., Ind. Eng. Chem. Res. 30, 575, 1991; Eckert, C., and Knutson, B., Fluid Phase Equil., 83, 93, 1993; McHugh, M., and Krukonis, V., Supercritical Fluid Extraction: Principles and Practice, Butterworth-Heinemann, Stoneham Mass., 1994). The addition of cosolvents can also greatly affect the solvating power of a SCF. They have high diftusivities and low viscosities, making them ideal candidates for use as solvents in heterogeneous reactions (See Tsekhanskaya, Y., Russ. J. Phys. Chem., 42, 532, 1968; Brennecke, J., AIChE Journal, 35, 9, 1989). Unlike a liquid solvent, a SCF can be easily and completely separated from the reaction products by depressurization. This is advantageous over processes utilizing organic solvents which require a costly step of solvent removal. Also, the solvents often cannot be removed completely, thus contaminating the resulting product. It is especially important to completely remove solvent when the reaction product is used as a reactant, for example, as a monomer in a polymerization reaction. Solvent residue will contaminate resulting polymers and often act as a plasticizer, limiting the utility of the polymer.
Using supercritical fluids as a solvent to produce polyster is well known and are described in U.S. Pat. No. 5,387,619 to Lee et al.; U.S. Pat. No. 3,480,587 to Porter et al.; and U.S. Pat. No. 3,052,711 to Glogan et al. Producing hydroxyethylester monomers from ethylene oxide and terephthalic acid is known from U.S. Pat. No. 3,520,853 to Munakata.
The present invention provides a process for synthesis of monoesters, such as mono-hydroxyethyl terephthalate, using a reaction catalyst or promoter, such as quaternary ammonium catalysts, to reduce the production of a condensation by-product, such as water. The process is conducted in the presence of a supercritical or near critical fluid, and, thus, does not require costly steps of separation of the products from the reaction media. Separated and recovered monoesters can be directly used as monomers in the synthesis of polyesters. Production of polyesters from monohydroxyesters, enables production of polyesters with the amount of water produced during polycondensation being half that produced in a conventional diol/diacid process. Therefore, less heat is required to remove the water, and processing costs are significantly reduced. In addition, a one-to-one ratio of reacting groups on the monomer simplifies the polymerization reaction.